In synchronous data communication systems, which use telephone lines as the transmission medium, the data are represented as a sequence of modulation states of a carrier signal. The modulation states are taken from a finite set of discrete amplitude and/or phase values which are called the transmission symbols. In the transmitters and receivers, the carrier is modulated and demodulated in modems according to given rules.
In a simple method, the sequence of binary data is separated into data words (e.g., 3-bit words) and each data word is directly represented by a corresponding transmission symbol (e.g. one of eight discrete phase values). More elaborate methods provide some redundancy to enable error detection and possibly correction. Even more sophisticated methods are known which employ a combined sequential encoding and modulation procedure in the transmitter and a particular sequential decoding process in the receiver so that, in the presence of signal distortion and noise, the original data sequence can be detected by the receiver with a larger margin for disturbances than if each data word were separately represented as a transmission symbol and separately detected.
In most transmission systems it is necessary to detect the end of a transmitted data sequence in order of the transmitted data. The exact moment when physical transmission ceases is difficult to detect for a modem receiver. Assume that the transmitter sends a sequence of transmission symbols from a given set (e.g. a set of 16 QASK symbols as shown in FIG. 3 of this specification), and an unmodulated carrier or zero energy after the end of transmission. Without a special end-of-transmission mechanism in the receiver it will be difficult to distinguish the "zero-symbol" from the symbols of the normal set (e.g. particularly from the innermost four symbols of the 16 QASK symbols of FIG. 3).
Turning the receiver off after detecting a single "zero-symbol" results in occasional erroneous receiver stops when the channel is noisy, or causes the modem to miss the end of transmission. Waiting for many "zero symbols" reduces the false-stop probability but does not lower the probability of missing the exact end of transmission. As a consequence, end-of-transmission control is usually left to a data link protocol which indicates end of transmission by a certain sequence of data bits transmitted. This solution has the disadvantage that end of transmission is protocol dependent and must be decided by the data terminal equipment that receives the data from the modem. The modem does not immediately detect the physical end of transmission and hence, e.g. in a half-duplex situation, it cannot immediately effect a line turn-around.